Evaluating the efficacy of leave-on cosmetic compositions to protect skin from pollutants

Abstract

Disclosed is a method of determining efficacy of a cosmetic composition to inhibit an atmospheric pollutant from contacting skin, comprising the steps of: (i) filling a sample holder of known dimensions with known amount of said composition to form a layer of defined thickness therein, where said sample holder is amenable to fluorescence-based assay; (ii) depositing, on said layer, a known amount of a model fine particulate matter which resembles an atmospheric pollutant at least in size and which comprises a material responsive to fluorescence microscopy which is excitable by a wavelength (λex) and which emits radiation (λem) where (λex)≠(λem); (iii) irradiating said composition and said model fine particulate matter by said λex and recording intensity of fluorescence at λem a plurality of times for a defined period during which said model fine particulate matter interacts with said layer; and, (iv) determining, corrected intensity of fluorescence at λem by the following equation; corrected Intensity=recorded intensity/average transmittance where said average transmittance is the average of the transmittance of the composition measured at λex and λem determined by Beer-Lambert law by measuring the absorbance at λex and λem, where the corrected intensity is inversely proportional to said efficacy of said composition.

The invention claimed is: 
     
       1. A method of determining efficacy of a cosmetic composition to inhibit an atmospheric pollutant from contacting skin, comprising the steps of:
 (i) filling a sample holder of known dimensions with known amount of said composition to form a layer of defined thickness therein, where said sample holder is amenable to fluorescence-based assay; 
 (ii) depositing, on said layer, a known amount of a model fine particulate matter which resembles an atmospheric pollutant at least in size and which comprises a material responsive to fluorescence microscopy which is excitable by a wavelength (λ ex ) and which emits radiation (λ em ) where (λ ex )≠(λ em ); 
 (iii) irradiating said composition and said model fine particulate matter by said A ex  and recording intensity of fluorescence at λ em  a plurality of times for a defined period during which said model fine particulate matter interacts with said layer; and, 
 (iv) determining, corrected intensity of fluorescence at λ em  by the following equation:
   corrected Intensity=recorded intensity/average transmittance 
 where said average transmittance is the average of the transmittance of the composition measured at λ ex  and λ em  determined by Beer-Lambert law by measuring the absorbance at λ ex  and λ em , where the corrected intensity is inversely proportional to said efficacy of said composition. 
 
 
     
     
       2. The method as claimed in  claim 1 , wherein said steps are also performed with a reference cosmetic composition, where said efficacy of said composition is the difference between efficacy of said cosmetic composition and said reference cosmetic composition. 
     
     
       3. The method as claimed in  claim 1 , wherein said model fine particulate matter resembles PM 2.5  or PM 10  at least in size. 
     
     
       4. The method as claimed in  claim 1 , wherein said fluorescence-based assay is fluorescence microscopy or fluorescence spectroscopy. 
     
     
       5. The method as claimed in  claim 1 , wherein said defined period is from 1 to 3000 minutes. 
     
     
       6. The method as claimed in  claim 1 , wherein slope (k) of a plot of intensity against time in the range of a linear increase is indicative of short-term efficacy of said candidate cosmetic composition and the nature of the plot reaching a plateau is indicative of its longer-term efficacy. 
     
     
       7. The method as claimed in  claim 6 , wherein data is collected at intervals of every 2 to 10 minutes starting from T=0. 
     
     
       8. The method as claimed in  claim 6 , wherein slope (k) of the linear fit of the data between T start  and T end , indicates the penetration speed of said model fine particulate matter into said layer, where T start  is the time when fluorescence intensity begins to increase, and T end  is the time when fluorescence intensity reaches plateau. 
     
     
       9. The method as claimed in  claim 6 , wherein from the slope (k) an extent of block efficacy is calculated using the formula:
   Block efficacy=1− k   cosmetic composition   /k   reference cosmetic composition ,
 
 where said block efficacy is indicative of how better the cosmetic composition is as compared to the reference cosmetic composition. 
 
     
     
       10. The method as claimed in  claim 1 , wherein said model fine particulate matter comprises a synthetic polymeric material, a natural polymeric material, a water-insoluble salt, a mineral, a metal, an alloy, glass or a mixture thereof. 
     
     
       11. The method as claimed in  claim 10 , wherein said model fine particulate matter is in the form of beads comprising polystyrene and a fluorescent material. 
     
     
       12. The method as claimed in  claim 1 , wherein said known amount of said composition is that amount which is sufficient to form, upon drying for 12 hours, a layer of 10 to 1000 μm in the sample holder. 
     
     
       13. The method as claimed in  claim 1 , wherein said sample holder is a cuvette or a microplate having plurality of sample wells arranged in a matrix where each well serves as a sample holder. 
     
     
       14. The method as claimed in  claim 1 , wherein said sample holder has a length of 0.5 to 2 cm. 
     
     
       15. The method as claimed in  claim 11 , wherein said fluorescent material absorbs and emits radiation of wavelength 400 to 800 nm.